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Impact of QCD Energy Evolution on Observables in Heavy-Ion Collisions

Heikki Mäntysaari, Björn Schenke, Chun Shen, Wenbin Zhao

Abstract

We study how the inclusion of energy dependence as dictated by quantum chromodynamic (QCD) small-$x$ evolution equations affects key observables in ultra-relativistic heavy-ion collisions. Specifically, we incorporate JIMWLK evolution into the IP-Glasma framework, which serves as the initial condition for a simulation pipeline that includes viscous relativistic hydrodynamics and a hadronic afterburner. This approach enables a consistent modeling of highly energetic nuclei across varying Bjorken-$x$ values, which are relevant for different collision energies and rapidity regions. In comparison to the standard IP-Glasma setup without small-$x$ evolution, we observe pronounced changes in particle multiplicities and spectral distributions, especially in smaller systems and at the highest available energies. We further explore effects on anisotropic flow observables and correlations between mean transverse momentum and elliptic flow. Our findings underscore the critical role of nonlinear QCD evolution in accurately modeling the early stages of heavy-ion collisions, as well as its implications for extracting transport properties of the quark-gluon plasma.

Impact of QCD Energy Evolution on Observables in Heavy-Ion Collisions

Abstract

We study how the inclusion of energy dependence as dictated by quantum chromodynamic (QCD) small- evolution equations affects key observables in ultra-relativistic heavy-ion collisions. Specifically, we incorporate JIMWLK evolution into the IP-Glasma framework, which serves as the initial condition for a simulation pipeline that includes viscous relativistic hydrodynamics and a hadronic afterburner. This approach enables a consistent modeling of highly energetic nuclei across varying Bjorken- values, which are relevant for different collision energies and rapidity regions. In comparison to the standard IP-Glasma setup without small- evolution, we observe pronounced changes in particle multiplicities and spectral distributions, especially in smaller systems and at the highest available energies. We further explore effects on anisotropic flow observables and correlations between mean transverse momentum and elliptic flow. Our findings underscore the critical role of nonlinear QCD evolution in accurately modeling the early stages of heavy-ion collisions, as well as its implications for extracting transport properties of the quark-gluon plasma.

Paper Structure

This paper contains 8 sections, 8 equations, 11 figures, 1 table.

Table of Contents

  1. Introduction
  2. IP-Glasma initial state with JIMWLK evolution
  3. Space-time evolution
  4. Results
  5. Heavy ion collisions
  6. O+O and Ne+Ne collisions
  7. p+Pb collisions
  8. Conclusions

Figures (11)

  • Figure 1: Charged hadron multiplicity distributions in Pb+Pb collisions at the LHC 5.02 TeV and 5.36 TeV, and Au+Au collisions at RHIC 200 GeV energies. The experimental data are taken from ALICE:2015juoALICE:2025cjnPHENIX:2004vdg.
  • Figure 2: Anisotropic flow $v_n$ in Au+Au collisions 200 GeV. The experimental data are from STAR:2016vqtSTAR:2017idkSTAR:2019zaf.
  • Figure 3: Collective flow $v_n$ in Pb + Pb collisions 5.02 TeV. The experimental data are from ALICE:2016ccg.
  • Figure 4: The $v_2\{4\}/v_2\{2\}$ ratio in Pb + Pb collisions at 5.02 TeV. The experimental data are from ALICE:2016ccg.
  • Figure 5: Model predictions for the charged hadron multiplicity distributions in O+O and Ne+Ne collisions at the LHC 5.36 TeV and at RHIC 200 GeV and 71 GeV energies. The preliminary ALICE data is from ALICE:MartaUrioni_IS2025
  • ...and 6 more figures